It is known that a significant amount of aerodynamic drag is created when a vehicle travels at velocities typical on a modern roadway. This, in large part, is due to areas of low pressure that act on rear surfaces of the vehicle. The low pressure becomes more pronounced as airflow over the vehicle separates from surfaces of the vehicle. The phenomenon of airflow separation is also well known in aircraft wing design and, in this case, causes the aircraft wing to stall.
Vehicles with blunt rear ends are especially affected by airflow separation starting at an abrupt transition to the near vertical rear end surfaces. The low pressure that the airflow separation causes is compounded by a relatively large area that the low pressure acts over compared with more streamlined vehicles.
The low pressure acting on the rear surfaces of the vehicle as it moves produces a force that resists forward motion of the vehicle. The force is opposed by the vehicle's engine and requires power that is typically produced by burning fuel. Any reduction in aerodynamic drag results in a reduction in fuel consumption.
In a period of high fuel prices, increasing fuel efficiency is a growing concern. Aerodynamic improvements are especially valuable since they can be combined with other improvements such as engine efficiency, reduced chassis weight, etc. Increasing the fuel efficiency also provides a valuable benefit of increasing a range that a given vehicle can travel between refueling stops.